Battery cells are just a commodity EV makers can purchase- they don't need to be able to engineer them in house. There are a lot more parts on an ICE car that pretty much need to be engineered by the vehicle manufacturer.
They don't need to engineer them, but for example BYD's vertical integration with batteries has been their superpower. Tesla once upon a time (~circa 2013) talked about building battery manufacturing "gigafactories" all over America but I don't know what the status of that is.
That is likely increasingly less relevant as battery technology matures. Back in 2013 the situation was entirely different, and off the shelf cells were all designed for things like laptop computers.
Panasonic (JP) designed the cells Tesla uses and produces half of them. (The rest are made by CATL, as far as I'm aware.) The "gigafactories" are mostly just Tesla providing facilities for Panasonic to use stateside to make cells, and then they do screwdriver assembly to make packs. Panasonic is also building more of their own factories in the US to serve other car companies.
Tesla doesn't have any competitive advantage in batteries.
